The Mysterious Moon of Makemake

Makemake and its moon as seen by HST
NASA, ESA, A. Parker and M. Buie (Southwest Research Institute), W. Grundy (Lowell Observatory), and K. Noll (NASA GSFC). Composite created by Carolyn Collins Petersen.

As we've explored in other stories, the outer solar system is really the new frontier of space exploration. This region, also called​ the Kuiper Belt, is populated with many icy, distant and small worlds that were once completely unknown to us. Pluto is the largest among them known (so far), and was visited in 2015 by the New Horizons mission. 

The Hubble Space Telescope has the visual acuity to make out tiny worlds in the Kuiper Belt. For example, it resolved the moons of Pluto, which are very small. In its exploration of the Kuiper Belt, HST spotted a moon orbiting a world smaller than Pluto called Makemake. Makemake was discovered in 2005 via ground-based observations and is one of five known dwarf planets in the solar system. Its name comes from the natives of Easter Island, who saw Makemake as the creator of humanity and a god of fertility. Makemake was discovered shortly after Easter, and so the discoverers wanted to use a name in keeping with the word.

The moon of Makemake is called MK 2, and it covers a pretty wide orbit around its parent body. Hubble spotted this little moon as it was about 13,000 miles away from Makemake. The world Makemake itself is only about 1434 kilometers (870 miles) wide and was discovered in 2005 via ground-based observations, and then further observed with HST. MK2 is perhaps only 161 kilometers (100 miles) across, so finding this tiny little world around a small dwarf planet was quite an accomplishment.

What Does Makemake's Moon Tell us?

When Hubble and other telescopes discover worlds in the distant solar system, they deliver a treasure trove of data to planetary scientists. At Makemake, for example, they can measure the length of the moon's orbit. That allows researchers to calculate MK 2's orbit. As they find more moons around Kuiper Belt objects, planetary scientists can make some assumptions about the likelihood of other worlds having satellites of their own. In addition, as scientists study MK 2 in greater detail, they can figure out more about its density. That is, they can determine whether it's made of rock or a rock-ice mix, or is an all-ice body. In addition, the shape of MK 2's orbit will tell them something about where this moon came from, that is, was it captured by Makemake, or did it form in place? Its history is likely very ancient, dating back to the origin of the solar system. Whatever we learn about this moon will also tell us something about conditions in the early epochs of solar system history, when worlds were forming and migrating. 

What's It Like on This Distant Moon?

We don't really know all the details of this very distant moon, yet. It will take years of observations to nail down its atmospheric and surface compositions. Although planetary scientists don't have an actual picture of the surface of MK 2, they know enough to present us with an artist's concept of what it might look like. It appears to have a very dark surface, likely due to discoloration by ultraviolet from the Sun and loss of bright, icy material to space. That little factoid comes NOT from a direct observation, but from an interesting side-effect of observing Makemake itself. Planetary scientists studied Makemake in infrared light and kept seeing a few areas that seemed warmer than they should be. It turns out what they may have been seeing as dark warmer patches were likely the dark-colored moon itself. 

The realm of the outer solar system and the worlds it contains have a lot of hidden information about what conditions were like when the planets and moons were forming. That's because this region of space is a veritable deep-freeze. It preserves ancient ices in much the same state they were when they formed during the birth of the Sun and planets. 

Yet, that doesn't mean things don't change "out there". On the contrary; there is plenty of change in the Kuiper Belt. On some worlds, such as Pluto, there ARE processes that heat and change the surface. That means that worlds DO change in ways that scientists are just beginning to understand. No longer does the term "frozen wasteland" mean that the region is dead. It simply means that temperatures and pressures out in the Kuiper Belt result in very different-looking and behaving worlds.

Studying the Kuiper Belt is an ongoing process. There are many, many worlds out there to find—and eventually explore. Hubble Space Telescope, as well as several ground-based observatories are the front line of Kuiper Belt studies. Eventually, the James Webb Space Telescope will be set to work observing this region too, helping astronomers locate and chart the many bodies that still "live" out in the solar system's deep freeze.